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Link between nighttime light exposure and mental health 

In a recent study published in the journal Nature Mental Health, an international team of researchers conducted a large-scale, cross-sectional analysis of sleep, light exposure, mental health, and physical activity to understand the association between light exposure during the day and night and the risk of self-harm and psychiatric disorders. 

The findings indicated that increased exposure to light during the night was associated with a higher risk of major depressive disorder, bipolar disorder, PTSD, psychosis, generalized anxiety disorder, and self-harm, while increased light exposure during the day was associated with a lower risk of psychiatric disorders. 

What is important about this study is not only the positive evidence of the link between light exposure and mental health but the scale of the study with over 86,000 individuals. 

The question “What is really going on here?” is raised very strongly and further research is now essential. If you contrast this study with the evidence of the blue light exposure hazard, what we don’t know yet is what properties of light are influencing this effect on mental health.  

We understand a lot about how certain spectra of light impacts the human circadian cycle for physiological functions and we at Circadacare are putting that science into everything we do to provide healthy lighting 24 hours per day. We see the benefits every day in the real world with improved wellbeing through better sleep/wake cycles and that, in itself, is a positive influence on mood and emotion. 

Minimising light exposure at night is baked-in to our circadian algorithms so perhaps we are already doing enough to mitigate mental health impacts. It will be vital to learn more about how this link really operates and we will be ready to provide lighting solutions that can address this major area of concern. 

About the study 

In the present study, the researchers used light-exposure data for over 86,000 individuals, recorded over seven days of light monitoring and actimetry to determine whether light exposure patterns during the day and at night were associated with various psychiatric disorders linked to disrupted circadian rhythms. The researchers hypothesized that an increase in light exposure during the day would be linked to better mood and a lower risk of psychiatric disorders, while an increase in night-time light exposure would conversely be linked to poorer moods and a higher risk of mental health disorders. 

The data for the study was obtained from the United Kingdom Biobank, in which participants had completed a week-long light monitoring and actimetry assessment. Individuals with unreliable accelerometry, light, or sleep data were excluded. Light exposure was measured using a triaxial accelerometer worn on the wrist and containing a silicon photodiode light sensor. 

An online mental health questionnaire was used to obtain information on mental health and psychiatric outcomes. The case/control psychiatric disorders were defined based on established guidelines. They included major depressive disorder, bipolar disorder including hypomania and mania, generalized anxiety disorder, post-traumatic stress disorder (PTSD), self-harm, and psychosis. 

Results 

The findings reported that, as hypothesized, an increase in light exposure during the night was linked to a higher risk of generalized anxiety disorder, major depressive disorder, PTSD, bipolar disorder, self-harm, and psychosis. Furthermore, increased light exposure during the day, independent of the light exposure at night-time, was linked to a lower risk of self-harm, psychosis, PTSD, and major depressive disorder. 

Additionally, these associations were significant even when the analyses were adjusted for covariates such as sociodemographic factors, physical activity levels, cardiometabolic health, and photoperiod. These associations were not only independent but also additive. Individuals in the quartile for brightest light exposure during the day were still at a higher risk of major depressive disorder if they had a higher light exposure at night-time. Conversely, those in the brightest light exposure quartile for night-time had a lower risk of major depressive disorder if they had a higher light exposure during the day. 

The sensitivity analyses reported that the associations were consistent even after accounting for factors such as urbanicity, shift work, cardiometabolic health, and sleep quality. The risk of major depressive disorder and self-harm was 30% higher for individuals in the brightest light exposure quartile for night-time. In comparison, those in the brightest light exposure quartile during daytime had a 20% lower risk of self-harm and major depressive disorder. 

Conclusions 

To summarise, the study examined the association between light exposure patterns during the daytime and night-time and the risk of psychiatric disorders. The findings indicated that increased exposure to light during the night was associated with a higher risk of major depressive disorder, bipolar disorder, PTSD, psychosis, generalized anxiety disorder, and self-harm, while increased light exposure during the day was associated with a lower risk of psychiatric disorders. The study suggested that modulating light exposure patterns may provide a simple, non-pharmacological option for improving mental health outcomes. 

Journal reference: 

Burns, A. C., Windred, D. P., Rutter, M. K., Olivier, P., Vetter, C., Saxena, R., Lane, J. M., Andrew, & Cain, S. W. (2023). Day and night light exposure are associated with psychiatric disorders: an objective light study in > 85,000 people. Nature Mental Health. https://doi.org/10.1038/s44220023001358, https://www.nature.com/articles/s44220-023-00135-8